Conventional medium voltage power inverter designs typically produce compromised results. For example, medium voltage drives and power supplies have been synthesized using high voltage devices, but these typically have high switching losses which adversely affects the output waveform harmonic content.
A conventional variable frequency medium voltage drive typically is employed to drive a medium voltage motor for the control of 5 MW to 75 MW loads or processes. The conventional medium voltage drive usually operates at medium voltages of 4.16 kV to 13.2 kV as operation at voltages substantially lower than 4.16 kV leads to excessive currents and power losses. While a conventional medium voltage drive can be manufactured using a standard high voltage power semiconductor, the selection and availability of standard high voltage power semiconductors are limited. In addition, conventional high voltage power semiconductors typically are severely lacking in switching speed, thus limiting the switching frequency. In addition, conventional medium voltage drives typically require specialized multi-level circuitry that tends to increase the dollar cost per kva of the drive.
Conventional low voltage drives of 400V-690V typically cost significantly less per kva than medium voltage drives due to higher production volume, technology maturity and market pressures. However, these conventional low voltage drives are not by themselves suited for controlling 5 MW to 75 MW loads or processes. Low voltage devices have been used in a series configuration, however, voltage sharing and balance during switching events is typically difficult and waveform harmonic content remain high. Conventional diode clamped and flying capacitor three-level converters allow a doubling of the dc bus and output voltage for a given switching device, but this voltage level is still limited by the device ratings. While the output harmonics are still excessive. Cascaded H bridge two-level converters, each fed by isolated direct current (DC) sources, have proven to be a suitable choice when a wide range of output voltage with high harmonic fidelity is desired. However, the number of cells required to synthesize medium voltages is excessive, and this directly correlates to an increase in the number of secondary transformer windings required to generate the dc source.
There is a need for a medium voltage inverter topology that is not reliant on series devices. Further, there is a need to increase the voltage and switching frequency capability of existing diode clamped and flying capacitor solutions. There is also a need to limit the number of cascaded cells in a cascaded two-level H bridge converter without impacting the voltage and switching frequency capability.
Thus, there exists a need for hybrid topology that increases the output voltage capability of each cascaded cell, thereby limiting the total number of cells, the associated rectifiers and transformer windings required to produce the isolated dc voltages.